Increasing global energy consumption and noticeable environmental pollution are making renewable energy more important. Today, a small percentage of total global energy comes from renewable sources, mainly hydro and wind power. However, global energy consumption is expected to expand by 58% between 2001 and 2025. As more countries ratify the Kyoto Accord, an international agreement to reduce greenhouse gas (GHG) emissions, new power generation capacity can no longer be met by traditional methods such as burning coal, oil, natural gas, etc. Also, these traditional sources are predicted to last only about 100 to 200 years in the world. Nuclear power plants have experienced safety problems and disposal of nuclear waste remains a serious issue. These issues increase the importance of renewable energy.
Energy from the wind, sun, water, waves, tides, etc., is renewable and essentially inexhaustible but the output from such sources is widely dispersed and generally sporadic, fluctuating dramatically with the weather and the seasons. Distributed generation (DG) technologies provide a potential solution of increasing electrical power generation capacity for renewable energy systems. Compared to large, centralized power grids, DG systems are usually small modular devices with increased security and reliability, and are generally close to electricity users, thus reducing the problems of power transmission and power quality issues due to very long transmission lines. DG systems often need dc-ac converters or inverters as an interface between their power sources and their typical single-phase loads. DG systems typically must deal with a wide range of input voltage variations due to the sporadic nature of the energy sources, which imposes stringent requirements on power inverters. Power inverters for small DG systems typically have the following requirements: (1) converting the variable incoming dc voltage into a fixed ac voltage with a fixed frequency; (2) ensuring output power quality with well controlled output frequency and low total harmonic distortion (THD); (3) providing electrical isolation and protection if necessary; and (4) low cost and high efficiency. DG systems are typically used to supplement the traditional electrical power grid and are often connected to the grid. In such cases, output power quality must meet specific standards, such as the interconnection requirements of IEEE 1547. For DG systems, the power grid source is strong enough to establish the output voltage waveform of inverters, thus the output current waveform and output power are often controlled objectives.
Traditional single-phase full-bridge inverters 100, as shown in FIG. 1 do not have the flexibility of handling wide ranges of input voltage. They often require large, heavy line-frequency step-up transformers 102 when handling low voltage dc inputs.
Examples of prior art two stage inverters are shown in FIG. 2, FIG. 3, FIG. 4, and FIG. 5.
Interest in buck-boost inverters has grown notably with the development of sustainable DG energy systems in recent years, because buck-boost inverters can handle a wide range of input voltages, both lower and higher than the desired ac output voltage. Examples of prior art two stage buck-boost inverters are shown in FIG. 6, FIG. 7, and FIG. 8.
Compared to two-stage buck-boost inverters, most of single-stage buck-boost inverters present a compact design with a good performance-cost ratio, but they suffer from low power capacity and limited operation range imposed to dc sources. Several S2B2 inverter topologies have been proposed in recent years. Examples of prior art single-stage buck-boost inverters are shown in FIG. 9, FIG. 10, FIG. 11, FIG. 12, FIG. 13, FIG. 14, and FIG. 15. Some of them still have higher component count and more complicated operations, even compared with a two-stage inverter, and thus compromise their benefits. Others limit their applications by either requiring split dc sources (FIG. 12) or imposing very high switching frequency (FIG. 13, FIG. 15) to demonstrate performance, or presenting low power ratings (FIG. 15).
Accordingly, an improved power converter having low power component count, wide input voltage range and improved performance, remains highly desirable.